Not invisible if you put it under a microscope.Google is your friend. Type in zygote and then look at all the images. A zygote is not invisible.
Did I need to say "invisible to the naked eye" to make it so you could understand it?
They use dyes, and colorations of images afterwards, to make their pictures look pretty. To see it NOT DIRECTLY, because it is not directly visible. Nothing to be seen without a microscope, no color. Not even black or grey.
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